1. Field of the Invention
This invention relates to a method for preparing polymers. More particularly, this invention relates to a method for preparing asymmetric radial polymers.
2. Prior Art
Heretofore, several methods have been proposed for preparing asymmetric radial polymers. As is well known in the prior art, radial polymers comprise three or more arms extending outwardly from a nucleus. The asymmetric radial polymers, generally, contain arms of at least two different polymers, which polymers may vary as to chemical composition, structure and/or molecular weights. Asymmetric radial polymers having arms of different molecular weights are sometimes referred to as polymodal polymers. A principal difference in the methods frequently used to prepare both asymmetric and polymodal radial polymers resides in the selection of a coupling agent which forms the nucleus of the radial polymer. The coupling agent may contain a fixed, though sometimes variable, number of functional sites such as the coupling agents taught in U.S. Pat. Nos. 3,281,383; 3,598,884; 3,639,517; 3,646,161; 3,993,613 and 4,086,298 or the coupling agent may itself be a monomer which polymerizes during the coupling reaction such as taught in U.S. Pat. No. 3,985,830.
In general, and when an asymmetric polymer is prepared using one of the methods heretofore proposed, a blend of polymeric arms is first prepared containing the various polymeric arms in the desired ratio and the blend of polymeric arms is then added to the coupling agent or the coupling agent is added to the blend of polymeric arms. These methods do, then, result in the production of a product having, on average, the desired number of each kind of arm in the asymmetric polymer. The real problem associated with producing asymmetric polymers in this fashion, however, is that the product obtained is in actuality a statistical distribution of all possible products which is represented by the equation: ##EQU1## for a polymer having the average composition (SI).sub.x (I).sub.y where each polymer component is designated as (SI.sub.1).sub.xi (I.sub.2).sub.yi wherein SI represents polystyrene-polyisoprene copolymer arms and I represents polyisoprene homopolymer arms on the radial polymer and quantities enclosed in brackets refer to molar concentrations.
For example, if one sought to produce an asymmetric radial polymer having three homopolymer arms and one copolymer arm using silicon tetrachloride as the coupling agent by the methods heretofore proposed, a blend of polymeric arms comprising both living homopolymers and living copolymers in a ratio of three to one would be combined with the silicon tetrachloride and the coupling reaction allowed to proceed to completion. The resulting asymmetric polymer would, of course, on average contain three homopolymer arms per copolymer arm. The actual product obtained would, however, be a blend of radial polymers, some of which contain four homopolymer arms and no copolymer arms, some of which contain three homopolymer arms and one copolymer arm (the desired product), some of which contain two homopolymer arms and two copolymer arms, some of which contain one homopolymer and three copolymer arms and some of which contain no homopolymer arms and four copolymer arms. The expected statistical distribution for an asymmetric radial copolymer having the average composition (SI)-X-I.sub.3 made in this manner, wherein X is silicon, is given in Table 1. To the extent that an asymmetric radial copolymer containing three homopolymer arms and one copolymer arm was particularly well suited for a particular end use application while radial polymers containing less than three homopolymer arms were not particularly well suited, the blend actually obtained would not, then perform as well as desired in this particular end use application.
TABLE 1 ______________________________________ Calculated Statistical Distribution of Polymer Components for the Asymmetric Radial Polymer Having a 3:1 Arm Ratio Polymer Component % Mole ______________________________________ (SI).sub.4 -X 0.4 (SI).sub.3 -X-I 4.7 (SI).sub.2 -X-I.sub.2 21.1 (SI)-X-I.sub.3 42.2 I.sub.4 -X 31.6 ______________________________________
Similarly, if one sought to produce an asymmetric radial polymer having two homopolymer arms and two copolymer arms using silicon tetrachloride as the coupling agent by the methods heretofore proposed, a blend of polymeric arms comprising both living homopolymers and living copolymers in a ratio of one to one would be combined with the silicon tetrachloride and the coupling reaction allowed to proceed to completion. The resulting asymmetric polymer would, of course, on average contain two homopolymer arms and two copolymer arms. The actual product obtained would, however, be a blend of radial polymers, some of which contain four homopolymer arms and no copolymer arms, some of which contain three homopolymer arms and one copolymer arm, some of which contain two homopolymer arms and two copolymer arms (the desired product), some of which contain one homopolymer and three copolymer arms, and some of which contain no homopolymer arms and four copolymer arms. The expected statistical distribution for an asymmetric radial copolymer having the average composition (SI).sub.2 I.sub.2 made in this manner is given in Table 2. To the extent that an asymmetric radial copolymer containing two homopolymer arms and two copolymer arm was particularly well suited for a particular end use application while radial polymers containing less than two copolymer arms were not particularly well suited (since this type of radial polymer would not form a mechanical network and would thus have very low strength), the blend actually obtained would not, then perform as well as desired in this particular end use application.
TABLE 2 ______________________________________ Calculated Statistical Distribution of Polymer Components for the Asymmetric Radial Polymer Having the Average Composition (SI).sub.2 I.sub.2 Polymer Component % Mole ______________________________________ (SI).sub.4 6.25 (SI).sub.3 I 25 (SI).sub.2 I.sub.2 37.5 (SI)I.sub.3 25 I.sub.4 6.25 ______________________________________
Recently, it has been discovered that narrower distributions of relative arm content in any given asymmetric radial polymer frequently does, indeed, lead to better performance in many end use applications. This is particularly important when an asymmetric radial polymer containing a certain arm ratio may give rise to deleterious properties in an application. The need, then, for an improved process for preparing asymmetric radial polymers offering better control of the relative distribution of polymer arms in the product is believed to be readily apparent.